A driver of a vehicle gazes at the front through a windshield, and performs driving while visually observing instruments on an instrument panel. That is, a line of sight moves to the front and lower instruments. If a driver can see instruments while looking at the front, there is no movement of the line of sight, and improvement of drivability can be expected. From this finding, a head up display device has been developed and put to practical use. As a conventional technique related to a head up display device, there is a technique disclosed in Patent Literature 1.
As shown in FIG. 1 of Patent Literature 1, the image light beam emitted upward from the HUD display (2) (the numerals in parentheses indicate the reference number described in Patent Literature 1, the same applies hereinafter) strikes the inner surface of the windshield (5) and is reflected and imaged in front of the driver (Patent Literature 1, paragraph number [0012]). When not in use, the optical path is blocked with the shutter (4) so that external light (sunlight) does not reach the HUD display (2). This makes it possible to prevent damage to the HUD display (2) (Patent Literature 1, paragraph number [0006]).
Since the shutter (4) and the driving means for driving the shutter (4) are necessary, the head up display device becomes expensive and becomes large. In addition, when in use, that is, during operation, since the shutter (4) is open, incidental sunlight cannot be prevented at this time. A structure capable of preventing the incidence of sunlight at all times, including during operation, is required.
Therefore, the inventors of the present invention proposed a head up display device, which took measures against sunlight without using a shutter. As a conventional technique relating to a head up display device taking measures against sunlight, there is a technique disclosed in Patent Literature 2.
As shown in FIG. 1 of Patent Literature 2, a reflection type polarizing film (21) (numerals in parentheses indicate the symbols described in Patent Literature 2, the same applies hereinafter) are interposed in the optical path. Due to the presence of the reflection type polarizing film (21), the temperature of the liquid crystal shell (16) does not rise even when sunlight (b) enters (Patent Literature 2, paragraph number [0013]). The reflection type polarizing film (21) is attached to the upper surface of the glass substrate (22).
According to Patent Document 2, it is possible to take measures against sunlight all the time. However, since the reflection type polarizing film (21) and the glass substrate (22) are indispensable, there is concern that the head up display device will be expensive and become large.
While miniaturization and cost reduction of the head up display device are required, a device capable of taking measures against sunlight without using a shutter or a reflection type polarizing film is desired.